Speaker
Description
Reducing the enormous heat flux on the divertor target is key to realizing future fusion reactors. One promising approach is to increase the neutral gas pressure near the divertor to promote strong plasma recombination. This leads to a detached plasma state, in which heat and particle fluxes to the target are significantly reduced. However, current understanding is insufficient to reliably predict detachment behavior in future devices, so it is essential to conduct complementary researches with experiments and simulations.
In the linear plasma device NAGDIS-II, which can produce a steady-state cylindrical plasma approximately 20 mm in diameter and ~2 m in length using DC discharge, an integrated transport code named “DISCOVER” is being developed to achieve high-accuracy simulations of detached plasma (H. Natsume et al., Physics of Plasmas 32, 022508 (2025)). As complementary experimental work, we have developed a two-dimensionally movable spectroscopic system, enabling measurement of plasma emission across a wide region by scanning both the axial and azimuthal directions. To eliminate the reflection effect from the wall, a reflection prevent plate was also installed.
This study measured wide-area helium emission distributions together with radial profiles of electron density and electron temperature at a fixed axial position by using a Thomson scattering system. By changing the discharge current and the gas flow rate, we created a large dataset. The emission distributions show a clear transition from an ionization-dominated attached plasma to a recombination-dominated detached plasma as the gas pressure increases. Using emission spectra and plasma parameters obtained at the same axial position, a machine-learning (neural network) model was trained. This was then applied to estimate wide-area plasma parameter distributions. Details of the full results will be presented at the presentation.